In recent years, global environmental problems such as global warming due to carbon dioxide, depletion of the ozone layer, and the like have been highlighted. For that reason, expectation for development of new energy, in particular, that for solar cells have become glowingly great. However, for popularization of the solar cells, there are a number of problems which must be solved. In particular, an improvement in conversion efficiency of the solar cells and a reduction in price are desired.
A CdS/CdTe solar cell uses CdTe which has an forbidden band gap (=1.44 eV) being suitable for a light absorption layer, and thus it is one of the solar cells which are expected to yield a high conversion efficiency.
In the manufacturing process of the CdS/CdTe solar cell, a cadmium telluride (CdTe) film is formed on a surface of a cadmium sulfide (CdS) film in general. As a method for forming the CdTe film, the close-spaced sublimation process which can produce the CdTe film of high quality is attracting attention. The close-spaced sublimation process is a kind of the vapor deposition process. A CdTe solar cell which has a conversion efficiency of the world highest level (15.8%) at present is obtained by this process. The close-spaced sublimation process is disclosed in, for instance, "HIGH EFFICIENCY CdS/CdTe SOLAR CELLS FROM SOLUTION-GROWN CdS FILMS" (The Conference Record of the 22nd IEEE Photovoltaic Specialists Conference (1991) Vol. 2, p.952) by T. L. Chu et. al., or the like. According to this process, a material for forming the CdTe film (hereinafter referred to as a source) and a substrate are so placed as to face each other, with a gap of about 0.5-5 mm, and heated under a reduced pressure. In this manner, the source is caused to sublime and then deposit on the substrate.
According to this process, since the sublimed source is rearranged and crystallized on the substrate placed at a short distance as long as the mean free path level, a CdTe film which has a high crystallinity is obtained. Further, since the treatment is performed under the reduced pressure, the film-forming speed is high.
However, the above-mentioned conventional close-spaced sublimation process has the following problems.
In general, in the close-spaced sublimation process, as a source, a CdTe powder placed on a dish-shaped container so as to cover over it is used. In the above-mentioned literature for instance, a commercially-available polycrystal of CdTe with a purity of 5N, or a powder produced by pulverizing a polycrystal ingot of CdTe obtained by directly implanting a dopant as one of the constituting element, is used as the source.
In addition to the fact that this process uses an expensive CdTe powder, it has a low utilization efficiency. In this process, it is difficult to evenly place as much amount of the source for forming the film just once for covering over the container. For that reason, the same source is repeatedly used for forming the CdTe films. It is difficult to control the CdTe powder or the source since it changes in particle size, powder density, stoichiometric ratio and the like by the sublimation, and hence, with the repetition of the film-forming, dispersions in thickness and in quality of the CdTe film increases gradually. Therefore, the obtained solar cell has a large dispersion in performance. For that reason, in order to make the dispersion in performance of the solar cell small, of the source placed for covering, only about 10% is actually consumed for the film-forming, and the rest is discarded without being used.
Further, according to this process, since the film is formed under the reduced pressure, an equipment must once be stopped when the source is exchanged. Therefore, the production efficiency is also low.
When a CdTe film is formed by this process, on a substrate which has a large area, a central part of the obtained CdTe film is made thicker than that of a peripheral part because the central part of the source is liable to trap a heat flowed from circumference, as opposed to the peripheral part where the heat is liable to escape. Therefore, the variation in thickness within the same film is large.
Moreover, according to this process, when the source is used repeatedly, the particle diameter of the material decreases gradually and the surface area of the material increases, and in addition, the surface temperature of the source rises because the particles combine together by sintering. Therefore, the thermal conductivity increases and the film-forming speed increases gradually. When the same source is used repeatedly still more and the remaining amount of the source decreases accordingly, pores are produced between and among the particles and the thermal conductivity of the source is gradually lowered, thereby to decrease the film-forming speed conversely. For that reason, even when the manufacturing is performed under the same conditions, the thickness of the obtained CdTe film varies for every film-forming processes.
As previously described, according to the conventional close-spaced sublimation process, it is difficult to obtain CdTe film having uniform quality and thickness within the same film and between a plurality of the films. For that reason, although the CdTe film theoretically has the forbidden band gap which is the most ideal for converting the solar light as a compound semiconductor, a CdTe film as an ideal p-type semiconductor has not actually been obtained by the conventional close-spaced sublimation process.
In order to improve the conversion efficiency of the CdTe solar cells, means of making the CdTe film a weak p-type one by diffusing copper or the like into the CdTe film from the electrode side has widely been used, as reported, for instance, by B. E. Mccandless et. al., in "A treatment to allow contacting CdTe with different conductors" (Conference record of the 24th IEEE photovoltaic specialists conference 1994 volume II, p. 107-110). However, according to this method, a solar cell having a satisfactory conversion efficiency has not been obtained.